1.Field of the Invention
The present invention relates to a method of collecting image data of an MRI (magnetic resonance imaging) for collecting image data of a plurality of sliced planes and to a magnetic resonance imaging (MRI) apparatus. More particularly, the present invention relates to a method of collecting image data of an MRI for collecting image data in synchronization with heart beat signals and to a magnetic resonance imaging apparatus.
2. Description of the Prior Art
An MRI employs an image employing a multi-slice technology (hereinafter called a multi-slice imaging) having an arrangement that a plurality of continuous imaged cross sections (hereinafter called sliced planes) are imaged repeatedly at predetermined repetition time (hereinafter called TR) so as to collect image data.
In MRI, it is required to restrict artifacts (a CSF (corebrospinal fluid) flow artifact and a bloodstream artifact and the like) generated due to pulsation of the heart when image data is collected. As a method of restricting artifacts of the foregoing type, there have been used a heart beat synchronizing method (an ECG synchronizing method) for synchronizing the timing of collecting image data with heart beats (or pulse waves) or a PPG (Peripheral Gating) method for synchronizing it with reflected signals from the bloodstream at the finger tip or the like. The foregoing synchronizing methods are characterized in that the timing of collecting image data is synchronized with reference pulses representing heart beats (hereinafter the reference pulses called R waves), the reference pulses being included in a signal, such as the heart beat signal or the reflected signal from the bloodstream at the finger tip or the like, which is in proportion to the movement of the heart (hereinafter the signal called the heart beat signal).
Since the multi-slice imaging performed by using the heart beat synchronizing method or the PPG synchronizing method is mainly employed when image data of, for example, a proton density weighted image data or a T2-weighted image data, is collected, a long TR is required. In the clinical operation, time longer than two heart beats is used as the TR. In this description, an operation of collecting image data by synchronizing with time corresponding to n (n 1) heart beats is called a synchronization with n heart beats.
FIG. 10 shows a specific procedure for collecting image data of a sagittal image of the spine by the multi-slice imaging employing the synchronization with two heart beats.
FIG. 10A shows an example of a sequence for collecting image data when the axis of abscissa stands for time t, in which R11, R12 and R21 represent R waves and R--R represents an interval between R waves. A pulse train shown in FIG. 10 indicates the timing of collecting image data. The synchronizing timing is expressed by arrow symbol .uparw.. FIG. 10B shows the positions (1 to 7 shown in FIG. 10B are called slicing positions hereinafter) on the sliced planes of a sagittal image of the spine. FIG. 10C shows the relationship between a sequential order (a sequential order of selective excitation of sliced planes) at the timing of collecting image data shown in FIG. 10A and the slicing positions shown in FIG. 10B.
The method of collecting image data employing the synchronization with two heart beats has an arrangement that a sliced plane (a slicing position 1 shown in FIG. 10B; hereinafter called a sliced plane 1) at the smallest slicing position is selectively excited in synchronization with a first R wave (R11) in the period TR to collect image data. Then, other sliced planes are excited at predetermined time intervals (hereinafter called interval I) to collect image data. Furthermore, the foregoing image data collection is repeated every TR (the time corresponding to two heart beats).
At this time, the sliced planes are selectively excited in a sequential order as shown in FIG. 10C, that is, every other sliced plane is sequentially selectively excited.
For example, the sliced plane 1 is selectively excited, and every other sliced plane at large slicing positions (odd number sliced planes) is sequentially selectively excited up to the maximum odd number sliced plane 7. Then, a sliced plane 2 at the minimum even number slicing position is selectively excited, and then every other sliced plane (even number sliced planes) is sequentially selectively excited.
When all sliced planes have been excited, synchronization with the reference pulse R21, after TR that is, (R--R)1+(R--R)2 (time corresponding to two heart beats)! has passed from the excitation of the sliced plane 1, is performed to selectively excite the sliced plane 1. Then, similar processes are repeated.
However, the foregoing method of collecting image data of a MRI employing the synchronizing method has the following problems: (1) The effect of restricting artifacts deteriorates due to deviation in the timing of collecting image data.
In order to restrict the artifacts, it is preferable that states of flow of image data to be collected every TR be the same. However, the interval (R--R) of the R wave is not strictly constant, thus resulting in the deviation of the timing of collecting image data at every data collection with respect to the time phase of the heart if a long time passes from the synchronizing timing. The timing of collecting image data at every data collection with respect to the time phase of the heart does substantially deviate immediately after the synchronizing timing.
An example of the deviation of the timing of collecting image data will now be described with reference to FIG. 11 (note that the waveform of the R wave is simply expressed by a pulse waveform). When the last two sliced planes D1 are selectively excited, the timing of collecting image data at the first data collection and the timing of collecting image data at the second data collection deviate from each other because the intervals of the R wave are different from each other ((R--R)2-(R--R)1=Ts). Moreover, a deviation in the timing of collecting image data at the selective excitation of the other sliced planes is present. The deviation in the timing of collecting image data with respect to the time phase of the heart while repeatedly selectively exciting the same sliced plane is one of the causes that deteriorate the effect of restricting artifacts. (2) The incomplete effect of restricting artifacts due to the problem (1) becomes critical for the sliced plane most clinically important, the artifact of which is required to be restricted most strictly.
In general, the artifact of a sliced plane of clinical importance must be restricted most strictly. If an image data of a sagittal image of the spine is collected, the most clinically important sliced plane is the central sliced plane (because a CSF flow artifact will be easily generated due to presence of many CSF flows on the sliced plane including the median line), and the second most important sliced sliced planes are planes adjacent to the central sliced plane. For example, among the sliced planes 1 to 7 of a sagittal image of the spine as shown in FIG. 12A, a central sliced plane 4 is the most clinically important (with mark S1). The secondly clinically important sliced plane is sliced plane 3 and sliced plane 5 (with mark S2) adjacent to the central sliced plane 4. The clinical importance degrades in a direction toward the periphery.
However, the image of a sagittal image of the spine has been usually performed by collecting data in such a way that a plurality of sliced planes are sequentially selectively excited starting from an end thereof.
FIG. 12B shows an example of a sequential order (which is the same as the sequential order shown in FIG. 10C) of the selective excitation employed in the image of a sagittal image of the spine. The foregoing sequential order causes the sliced planes 3 and 5 to be selectively excited immediately after the synchronizing timing to collect image data. However, image data of the sliced plane 4, the most clinically important is collected at the sixth order. Thus, the synchronizing effect deteriorates due to the problem (1). That is, the foregoing sequential order of the selective excitation encounters deterioration in the effect of restricting artifacts of the sliced plane, the artifact of which is intended to be restricted most strictly.